Optical fibers are being widely utilized in various communication systems such as local area networks, computer networks and standard subscriber loops. While optical fibers are capable of carrying a large amounts of information, the fibers lose their attractiveness unless there is some way to extract the information from the fiber for the end-user.
Couplers and, in particular, directional couplers have been developed exactly for this purpose. Directional couplers such as those described in U.S. Pat. Nos. 4,307,933, 4,317,614 and 4,431,260 include primary and secondary optical fibers each having polished flat surfaces in contact in the presence of an index matching fluid to accomplish evanescent directional coupling. In these couplers, optical signals are coupled from the primary fiber into the secondary fiber and the optical signals propagate in the secondary fiber in the same direction in which the signals travelled in the primary fiber. Also, the coupling is broadband in nature. That is, some percentage of each and every optical signal in the primary fiber are coupled into the secondary fiber regardless of mode or wavelength. Hence, directional couplers as described above lack wavelength selectivity which is necessary for applications such as multichannel wavelength division multiplexing and coherent communication systems.
Wavelength selective optical couplers have been predicted and described for embedded (side-by-side) channel waveguides devices by N. Imoto in Jour. of Lightwave Tech., Vol. LT-3, No. 4, pp. 895 et seq. (1985). The couplers proposed therein are contradirectional because optical signals propagating in the primary waveguide at a particular narrow set of wavelengths are coupled to the secondary waveguide by a grating fiber to propagate in opposite (contrary) direction therein. It should be noted that Imoto stresses the importance of fabricating the grating filter over the secondary (output) waveguide to reduce undesired coupling between the embedded waveguides. While this type of coupler is applicable to the wavelength division multiplexing problem, it requires a complete change of transmission medium from fiber to embedded waveguide and then back to fiber and, in making those changes, it imposes significant insertion losses on the communication system as a whole.